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⌂ ∞ ▽ Aviator on the BBC Micro

Flight model: ProcessLanding (Part 7 of 7)

       Name: ProcessLanding (Part 7 of 7)                            [Show more]
       Type: Subroutine
   Category: Flight model
    Summary: We have successfully touched down without crashing, so process the
             effects of landing on the plane
  Deep dive: Take-offs and landings

    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file




 This part processes a successful landing.



.clan17

 LDY zRotationLo        \ Set (A Y) = zRotation
 LDA zRotationHi

 JSR Multiply8x16-6     \ Set (G W V) = (A Y) * R
                        \             = zRotation * yVelocity / 2
                        \
                        \ Note that the value of yVelocity used is the original
                        \ value, rather than the halved value that we set in
                        \ part 5 if this is an emergency landing

 SEC                    \ Negate (G W), so (G W) = -zRotation * yVelocity / 2
 JSR Negate16Bit

 LDA #0                 \ Set T = 0 to act as a high byte in (T G W)
 STA T

 LDA G                  \ Set (T A W) = (T G W)

 BPL clan18             \ If the high byte in G is positive, jump to clan18 to
                        \ skip the following instruction

 DEC T                  \ Decrement T to &FF so it has the correct sign for
                        \ (T A W), so we now have:
                        \
                        \   (T A W) = -zRotation * yVelocity / 2

.clan18

 LDX #1                 \ We now want to halve (T A W) twice, so set a shift
                        \ counter in X

.clan19

 LSR T                  \ Set (T A W) = (T A W) / 2
 ROR A
 ROR W

 DEX                    \ Decrement the shift counter

 BPL clan19             \ Loop back until we have shifted right by two places,
                        \ so now we have the following:
                        \
                        \   (A W) = (A W) / 4
                        \         = (-zRotation * yVelocity / 2) / 4
                        \         = -zRotation * yVelocity / 8
                        \
                        \ We can ignore the value of T as it was only used to
                        \ feed bits of the correct polarity into the high byte


 STA zTurnTop           \ Set (zTurnTop zTurnHi) = (A W)
 LDA W                  \                       = -zRotation * yVelocity / 8
 STA zTurnHi            \
                        \ so this applies a turn moment to the plane that is in
                        \ the opposite direction to the current roll rotation,
                        \ and which is proportionate to the speed, so if we come
                        \ in fast and at a large roll angle, then the plane will
                        \ be turned fast in the opposite direction

 LDX #&EC               \ Set (zRotationHi zRotationLo) = 0
 JSR ResetVariable

 LDA xRotationHi        \ If the high byte of the plane's rotation around the
 BPL clan20             \ x-axis is positive, then the plane is tilting
                        \ backwards, so jump to clan20 to skip the following

 LDA ucStatus           \ If ucStatus is non-zero, then the undercarriage
 BNE clan20             \ is down, so jump to clan20

 LDX #&EA               \ If we get here then the undercarriage is up and the
 JSR ResetVariable      \ plane is tilted forwards as we're landing, so we set
                        \ (xRotationHi xRotationLo) = 0 to belly-flop the
                        \ plane forwards onto the ground, so it lands (and
                        \ slides) horizontally along the ground

.clan20

 LDA R                  \ If R >= 12, i.e. yVelocity >= 24, jump to clan21 to
 CMP #12                \ return from the subroutine
 BCS clan21

 LDA yPlaneLo           \ If yPlaneLo <> yLandingGear, jump to clan21 to return
 CMP yLandingGear       \ from the subroutine
 BNE clan21

 LDA #1                 \ If we get here then the vertical velocity is <= 24,
 STA onGround           \ and the distance between the plane and the ground is
                        \ equal to the distance from the cockpit to the bottom
                        \ of the plane... in other words, we are exactly on the
                        \ ground and moving very slowly, so set onGround = 1 to
                        \ denote that we are on the ground

.clan21

 RTS                    \ Return from the subroutine